Usually, polyolefin polymerization processes make use of a purge bin after the reactor vessel to remove unwanted volatiles from the polymer resin. The purge bin is a vessel where a resin mixture enters the upper portion of the vessel and is subjected to purging gas through ports or openings at the bottom of the vessel and possibly along the sides and other areas of the vessel to remove the volatiles through a purging effect.
However, simply piping a purging gas into the resin without taking into account solids flow distribution, distribution of the purging gas through the resin, resin flow patterns, and any potential heating effect of the purging gas can result in damage to the resin and possibly lead to poor or non-commercially viable polymer products. In addition, the time that the resin is exposed to the purging gas may also affect the degree to which the volatiles are removed from the resin.
The interface between the resin and the purge bin along with any associated projections (such as pipes, tubes, supports, etc., that may protrude into the resin flow path) may also have an effect on the flow rate of the resin through the purge bin. Since volatiles may be purged from the resin at a rate dependent on the time of contact between the resin and the purge gas, any uneven flow pattern of the resin (i.e., slower or faster resin flow) can affect the amount of volatiles that are purged. Accordingly, the amount of volatiles removed differs from one portion of the purge bin to another dependent on the flow pattern of the resin.
Insert assemblies through which gas may be added and/or removed from the flow path of the resin are often included in the purge bin to address some of these problems with mass flow in the purge bin as well as gas distribution within the solids bed. One example of an insert assembly includes an inverted cone with one or more cylindrical member sections beneath the inverted cone whose purpose is to achieve a near constant velocity profile of the resin traveling downward in the purge bin. Other types of insert assembly may also be included in the purge bin. However, by inclusion of the insert assembly in the purge bin, the bin diameter may be undesirably increased thus increasing bin cost. This is because the diameter of the purge bin is a function of the area of the insert assembly. This relationship between bin diameter and insert area occurs because a maximum superficial velocity must not be exceeded without unacceptably high resin entrainment occurring in the case of removing gases from the solids bed. Entrainment of resin in the withdrawn gas may be undesirable for downstream equipment and may result in piping fouling. Moreover, when introducing gas into the bed of solids, it also is undesirable to locally fluidize the solids as this may result in gas maldistribution, an interruption of solids flow and segregation.